Actually, I take that back. I just tried adding an INTERNAL{} module to the cfg for the hitchhiker, and it works just fine. It even makes it so the crewmember's portrait is shown in the flight scene. I guess all we need now is for someone to make a model for that interior.

Well, it involves making 3d models with software such as Blender or 3DS Max, then making texture files with image manipulation software (anything from MS Paint to GIMP to Photoshop), then doing a little bit of work with the Unity editor, which you can download for free. My own toolkit is Blender and GIMP, because both are free software.